The present invention relates to microbiological processes for the oxidative pretreatment of refractory gold and base metal ores and concentrates, and, more specifically, to novel bacterial cultures and inoculums strongly resistant to thiocyanate toxicity which are useful in such processes. The invention further relates to methods of developing and using these resistant bacterial strains.
Precious metals are found throughout the world as an ore within the Earth""s crust on the crust surface and dispersed in bodies of water. The precious metal is nearly always in an unrefined state intimately associated with other minerals such as sulfur in the form of arsenopyrite or pyrite. To extract the metal, ore must be processed to remove contaminating minerals such as sulfur, carbon and iron. A commonly used processing technique is cyanidation which involves, quite simply, leaching the ore with cyanide. The cyanide leaches the ore, releasing the precious metal from its association with the gangue minerals. Released metal leaches into a liquid phase from which it can be recovered.
Gold ores are categorized into two typesxe2x80x94free milling and refractoryxe2x80x94depending on their refractoriness to cyanidation treatment. Free milling ores generally have a low sulfur content and are most often processed by simple gravity techniques or direct cyanidation. Refractory ores, having a higher sulfur content, are difficult to process due to a high excess content of metallic sulfides, such as pyrite, arsenopyrite and other matter, and require more complex extraction methods. One of the most common of such measures is oxidation.
Oxidation of refractory ores involves a pretreatment step in which the ore is subjected to well-known roasting or pressure-oxidation techniques, typically in conjunction with a pre-concentration process. Increasingly, biooxidation is being used as the pretreatment of choice in substitution of these other more traditional oxidation processes. In biooxidation, the metal sulfides in ore are oxidized in a microbial pretreatment step, prior to the cyanidation step. Specifically, the bacteria oxidize both iron and sulfur under acidic conditions. Oxidation of iron sulfide particles causes the solubilization of iron as ferric ion and sulfide as sulfate ion. This liberates the encapsulated precious metal and makes it amenable to a leaching agent, such as cyanide. The precious metal is subsequently recovered from the oxidized materials by cyanidation, carbon-in-leach of thiosulfate leaching processes.
The adaptation of bacteria in the biooxidation process to recover precious metals from refractory ores has been previously described in a number of variations. For example, one method involves oxidizing multi metallic sulfide ores using a combination of chemical/biological leaching process and at least three different types of bacteria (U.S. Pat. No. 4,987,081). Bacterial cultures of Thiobacillus thiooxidans, Thiobacillus ferrooxidans and Leptospirillum ferrooxidans are first adapted to high dissolved arsenic concentrations and low pH by subjecting the cultures in a solution containing dissolved arsenic to successive incremental concentrations of arsenic while operating in a continuous mode.
Another process involves the biological oxidation of sulfide in sulfide-containing gold ore followed by cyanide leaching (U.S. Pat. No. 5,006,320). This method involves a further processing step for aerating microorganisms during the oxidation step followed by a subsequent extraction of the metal value from the biooxidized ore.
Biooxidation is not limited to the treatment of gold ores. A related method for producing nickel from sulfide ore involves oxidation by heap leaching (Canadian Patent No. 2,155,050). According to this method, nickel ore, which contains a substantial amount of iron, is subjected to a biological oxidation step and separated from iron into an eluate solution. Nickel is removed from the solution by solvent extraction or by use of an ion exchange resin and subsequent electrowinning of the ferronickel.
Metals can also be recovered from refractory sulfide ores by first separating the crushed ore in to a fines and a coarse fraction (U.S. Pat. No. 5,573,575). A heap is formed from the coarse fraction and a concentrate is produced from the fines. The concentrate is then added to the heap for biooxidation.
Alternatively, biooxidation of sulfides in the mineral ores may be done by forming particulates. A heap of particulates is formed and a leaching solution is circulated within the heap (U.S. Pat. No. 5,246,486). A variation on this technique involves polymer agglomeration to aid in the removal of particulates from the metal ore (U.S. Pat. No. 5,332,559).
Metals can also be recovered from a refractory sulfide ore by first separating the clays and fines from the crushed ore, and forming a heap from the crushed refractory ore (U.S. Pat. No. 5,431,717). If there is a sufficient amount of precious metal in the separated clays and fines, these materials are further processed.
Methods for the biooxidation of refractory carbonaceous or carbonaceous-sulfidic ore material using a specific carbon-deactivating microbial consortium have also been used with varying degrees of success (U.S. Pat. No. 5,244,493).
Preg-robbing by carbon and carbon-containing compounds is also a major problem interfering with efficient recovering of metals from refractory ores. One process to overcome this problem uses leaching with a thiosulfate lixiviant to selectively remove the metal (U.S. Pat. No. 5,354,359). This process involves contacting particulates containing precious metal and preg-robbing carbonaceous components with a thiosulfate lixiviant solution forming stable precious metal thiosulfate complexes. The lixiviant solution is recovered after it has had time to become loaded with the metal in the ore material.
Leaching has also been used to remove copper from copper sulfide-containing ore (U.S. Pat. No. 4,571,387). According to this process, ore is ground and mixed with an aqueous acid-leaching medium containing sulfide-oxidizing bacteria, a bacterial nutrient and a catalytic amount of silver. Carbon dioxide and oxygen are provided as well as a bacterial compatible acid. The basic leaching process has been enhanced to increase the leaching rate of a mineral when the mineral is characterized by the tendency to form a reaction product layer during leaching (U.S. Pat. No. 4,343,773). A particulate modifier such as carbon is mixed with the mineral before leaching and selectively alters the characteristics of the reaction product layer.
Prior to incurring the substantial costs inherent in scaling up to biooxidize a particular ore, the ore under consideration typically is batch tested to determine if it is suitable for biooxidation. However, conventional testing procedures can take as long as six months to complete due to the time needed for adaptation of the bacteria and the lag phase between inoculation and the onset of oxidation.
A number of intermediate oxidation products of sulfur including elemental sulfur, polymeric sulfur, sulfite, thiosulfate and polythionates are generated during biooxidation, particularly if oxidation is incomplete. Many of these compounds will react with cyanide in the gold extraction process to form thiocyanate, which is a major cyanide consumer. If a cyanide destruction process is not incorporated as part of the treatment plant, the thiocyanate is discharged with the process tailings. In conventional gold extraction processes, tailings solution containing thiocyanate is often recycled to the process. However, thiocyanate is toxic to the bacteria employed in biooxidation at relatively low levels of between 5 ppm and 25 ppm.
Thus, in systems utilizing microbial biooxidation as the method for oxidizing the ore, tailings solution cannot currently be recycled upstream of the biooxidation process.
In addition, the need for high quality water in biooxidation places further constraints on the process, both in terms of the requirement for large quantities of fresh water and the inability to re-use tailings solution.
The present invention overcomes the problems and disadvantages associated with current strategies and designs and provides novel bacterial cultures and inoculums which are resistant to thiocyanate, and, thus, can be used in processes where the bacteria may be exposed to thiocyanate, such as those where tailings solutions are reused. In addition, for materials of specific mineralogical content, the inoculums of this invention may be used to facilitate novel processes involving simultaneous biooxidation and co-extraction of gold using a thiocyanate extraction route.
The present invention relates to bacterial inoculums or cultures that are capable of effectively oxidizing refractory ores while simultaneously being resistant to thiocyanate. The inoculums of this invention prevent collapse of the process during inadvertent use or intentional recycling of tailings solution containing thiocyanate. Such bacteria also allow greater environmental acceptability by permitting re-use of tailings solution, which would have a positive impact on the process water balance and allow cost reductions associated with a reduction of fresh water requirements.
One embodiment of the invention is directed to methods for isolating a thiocyanate-resistant bacterial culture. These methods involve inoculating a tail sample containing a concentration of thiocyanate with a population of biooxidative bacteria; culturing the population in nutrient medium contain a first concentration of solids for a first period of time; increasing the solids concentration of the medium to a second concentration of solids and culturing the population for a second period of time; increasing the solids concentration of the medium to a third concentration of solids and culturing the population for a third period of time; and harvesting the population of bacteria.
Another embodiment of the invention is directed to methods for recovering a metal from a refractory ore. These methods involve biooxidize the ore using the bacteria of the present invention, and leaching the metal in an acidic thiocyanate solution. In a preferred embodiment, the biooxidize and the leaching are performed substantially simultaneously.
A further embodiment of the present invention is directed to bacterial inoculums and cultures that are resistant to thiocyanate and to bacterial inoculums and cultures that are isolated by the methods of this invention.
Other embodiments and advantages of the invention are set forth, in part, in the description which follows and, in part, will be obvious from the description and may be learned from the practice of this invention.
As embodied and broadly described herein, the present invention is directed to novel bacterial cultures and inoculums, and to methods of developing and using these strains. The bacteria of the present invention are thiocyanate-resistant and useful in the biooxidative pretreatment of refractory gold ores and other refractory ores. The use of such bacteria confers a number of technological advantages relating to improvements in operating flexibility, costs and environmental acceptability.
The economics of biooxidation processes are related to oxidation requirements. Biooxidation test work is aimed at determining the minimum amount of sulfide oxidation required in order to achieve acceptable metals extraction. The extent of sulfide oxidation is in turn dependent on the mineralogical composition of the material to be treated and the relative distribution of the associated metal values.
For example, the gold in refractory gold ores or concentrates often occurs as a colloidally dispersed phase within an arsenopyrite or arsenian matrix. In this case, complete destruction of the arsenopyrite matrix is generally required to recover the gold. However, gold also commonly occurs as micron-sized particles along the grain boundaries and interstices within pyrite, in which case it is generally more accessible. Depending on the mineralogy and gold distribution, acceptable gold recoveries are usually obtained at total sulfide oxidation levels of somewhat less than 100%.
However, at these lower oxidation levels, sulfur and other intermediate sulfur compounds can form, which react with cyanide to form thiocyanate. These compounds include sulfur, polymeric sulfur, sulfite, thiosulfate, and polythionates. Further, it has been found that the bacteria generate an extracellular capsule of colloidal sulfur, which the bacteria use as a store of metabolic energy. Also, the bacteria produce the enzyme rhodanese, which is employed in their internal metabolic pathways, that catalyzes the production of thiocyanate through the formation of an intermediate rhodanese-sulfur complex.
In biooxidation processes, these deleterious compounds, and, to a large extent, the bacteria, are removed before cyanidation by either rinsing with water or by washing in a counter current decantation circuit. Because the washing step is never 100% efficient, a small but significant portion of these materials report to the cyanidation process, resulting in generation of thiocyanate. This formation of thiocyanate in the gold recovery process is by far the largest consumer of cyanide and the primary source of thiocyanate.
Chemical reactions illustrative of the formation of sulfur, sulfur intermediates and thiocyanate in biooxidation-based gold extraction systems are indicated below.
S2xe2x88x92+3H2Oxe2x86x92SO32xe2x88x92+6H++6exe2x88x92
S0+O2+H2Oxe2x86x92SO32xe2x88x92+2H+
SO32xe2x88x92+H2Oxe2x86x92SO42xe2x88x92+2H+2exe2x88x92
2S2O32xe2x88x92+2exe2x88x92xe2x86x92S4O62xe2x88x92+2exe2x88x92
2S2O32xe2x88x92+2exe2x88x92xe2x86x92SO32xe2x88x92+S2xe2x88x92
S4O62xe2x88x92+H2Oxe2x86x92SO42xe2x88x92+S2O32xe2x88x92+2H+
S2O32xe2x88x92+CNxe2x86x92SO32xe2x88x92+SCNxe2x88x92
4Fe7S8+6SO2+10H2SO4xe2x86x9214Fe2(SO4)3+10H2O
Fe7S8+7Fe2(SO4)3xe2x86x9221FeSO4+8S0 
4FeAsS+5O2+4H2SO4xe2x86x924HAsO2+4FeSO4+4S0+2H2O
CuFeS2+O2+2H2SO4xe2x86x92CUSO4+FeSO4+2S0+2H2O
2S2xe2x88x92+3O2xe2x86x922SO32xe2x88x92
2S2xe2x88x92+4SO32xe2x88x92+6H+xe2x86x923S2O32xe2x88x92+3H2O
S2xe2x88x92+3SO32xe2x88x92+6H+xe2x86x923S4O62xe2x88x92+3H2O
S0+SO32xe2x88x92xe2x86x92S2O32xe2x88x92
S4O62xe2x88x92+SO32xe2x88x92xe2x86x92S2O32xe2x88x92+S3O62xe2x88x92
S4O62xe2x88x92+S2O32xe2x88x92xe2x86x92S5O62xe2x88x92+SO32 
4S4O62xe2x88x92+6HOxe2x88x92xe2x86x925S2O32xe2x88x92+2S3O62+3H2O
2S5O62xe2x88x92+6HOxe2x88x92xe2x86x925S2O32xe2x88x92+3H2O
S+CNxe2x88x92xe2x86x92SCNxe2x88x92
2S2O32xe2x88x92+O2 2CNxe2x88x92xe2x86x925SCNxe2x88x92+2SO42xe2x88x92
For complete oxidation, the sulfur is oxidized through to sulfate. However, at less than complete sulfide oxidation, there are a number of alternative routes by which thiocyanate can be generated in the system.
As noted, a number of intermediate oxidation products of sulfur are generated during biooxidation, particularly if oxidation is incomplete. Many of these compounds will react with cyanide in the gold extraction process to form thiocyanate. Thiocyanate is toxic to the bacteria conventionally employed in biooxidation at relatively low levels of between 5 ppm and 25 ppm. Consequently, tailings solution cannot currently be used with conventional bacterial strains upstream of the biooxidation process.
The bacteria used in biooxidation processes are generally referred to as aerobic, acidophilic, thermophilic, auto-chemolithotrophs; that is, they prefer warm acid and aerobic conditions in which to thrive and grow by metabolizing mineral substrates. The types employed in biooxidation fall into two broad categories: mesophiles and moderate thermophiles. Mesophilic cultures usually comprise a mixed consortium of Thiobacillus ferrooxidans, Thiobacillus thiooxidans and Leptospirillum fenooxidans. Moderate thermophiles usually comprise Sulfobacillus thermosulfidooxidans, Metallosphera sedula and bacteria of the sulfolobus type.
The present invention comprises a novel mixed culture of thiocyanate-resistant, mesophilic bacteria. This mixed culture is derived by inoculating a tail sample containing thiocyanate with a population of biooxidative bacteria. The population is cultured in a nutrient medium containing a concentration of solids. The solids concentration of the medium is sequentially increased and the bacteria cultured over a period of time in each concentration. OK medium is added to the media as needed to make up for evaporation losses. Finally, the adapted population of thiocyanate-resistant bacteria may be isolated or harvested to use as an inoculum in a biooxidation process.
Accordingly, one embodiment of the invention is directed to a method for isolating a thiocyanate-resistant bacterial culture comprising the steps of: inoculating a tail sample containing a concentration of thiocyanate with a population of biooxidative bacteria; culturing the population in a nutrient medium containing a first concentration of solids for a first period of time; increasing the solids concentration of the medium to a second concentration of solids and culturing the population for a second period of time; increasing the solids concentration of the medium to a third concentration of solids and culturing the population for a third period of time; and harvesting the population of bacteria.
In a preferred embodiment, the biooxidative bacteria comprise one or more species of bacteria selected from the group consisting of T. Ferrooxidans, T. Thiooxidans and L. Ferrooxidans, the T. Ferrooxidans 2,000 period of time is between 20 and 30 days, and the third period of time is between 10 and 20 days. The tail sample preferably comprises a pyritic concentrate that has been leached with cyanide for the recovery of gold. The initial thiocyanate concentration is preferably between 500 and 600 ppm. The nutrient media preferably comprises OK medium.
Still another embodiment is directed to bacteria isolated by the foregoing method. The bacterial strain of the present invention has a demonstrated tolerance to thiocyanate which is 23 times that reported elsewhere in the literature. Consequently, the bacteria of the present invention provide technological advantages in biooxidation processes including: improved operating flexibility, increased environmental acceptability, and operating cost reduction.
Gold can be leached in acidic thiocyanate solutions. The bacterial inoculum of the present invention, which is highly resistant to thiocyanate, allows an oxidation/co-extraction process based on thiocyanate. Such a process has major advantages over existing protocols, since the oxidation of sulfides and extraction of gold can be carried out simultaneously as opposed to sequentially as is the current practice. Such a process reduces capital costs by eliminating a step in the conventional process and lowers operating costs associated with the use of cyanide, lime, power and maintenance of environmental protection.
The oxidation potential required to oxidize gold metal to gold-I in thiocyanate solutions is about 600 mV and that required to reduce ferric ions is about 800 mV. This indicates ferric ions are a suitable oxidant for gold. Thiocyanate is thermodynamically unstable in acid media (pH of 1-3) but oxidation to cyanide and sulfate is extremely slow. Therefore, the conditions under which biooxidation is normally conducted are conducive to the extraction of gold by thiocyanate. The appropriate reactions are indicated below.
Fe(SCN)4xe2x88x92+Auxe2x86x92Fe2++Au(SCN)2xe2x88x92+2SCNxe2x88x92
3Fe(SCN)4xe2x88x92+Auxe2x86x923Fe2++Au(SCN4xe2x88x92+8SCNxe2x88x92
The gold thiocyanate complex can subsequently be recovered with either a strong base resin or carbon.
Ferric ions form strong complexes with thiocyanate, which at low concentrations can reduce the oxidizing potential of ferric and at the same time reduce the concentration of free thiocyanate. Optimal conditions for the extraction of gold by thiocyanate in biooxidative systems, in terms of concentration ranges, redox potential, pH and temperature are under investigation.
Accordingly, another embodiment is directed to a method for recovering a metal from a refractory ore comprising the steps of biooxidize the ore using the bacteria of the present invention, and leaching the metal in an acidic thiocyanate solution. In a preferred embodiment, the steps of biooxidize and leaching may occur substantially simultaneously. The pH of the solution is preferably from about 1 to about 3. The temperature of the solution is preferably from about 25xc2x0 C. to about 70xc2x0 C. The solution preferably has a concentration of thiocyanate of between about 1,000 and 2,000 ppm. The metal being recovered is preferably gold; however, as will be clear to those of ordinary skill in the art, the present invention can be used to recover other precious metals and materials such as nickel, copper and cobalt.